1. Field of the Invention
This invention relates to ultrasonic measuring apparatus and, more specifically, to an ultrasonic-signal transit-time measuring instrument wherein the difference between the transit time of the ultrasonic signal and the time for a counter to count a predetermined number of half cycles of the output signal from a voltage-controlled oscillator is detected and such information is used to vary the frequency of the oscillator until such time difference is zero, the transit time for the ultrasonic signal being determinable from such varied oscillator frequency.
2. Prior Art
Ultrasonic-signal transit-time measuring devices have been described in the co-pending applications set forth herein and, in flowmeter systems, are described in an article entitled Fuji Ultrasonic Flowmeter appearing at pages 29 to 38 of the journal Fuji Jiho, volume 48, No. 2, published by Fuji Electric Company, Ltd. of Tokyo, Japan. In such systems, not utilizing this invention, an ultrasonic acoustic signal emanating from a transmitting transducer is received by an acousto-electric transducer after it passes through the medium being measured.
The electrical signal from this latter transducer is amplified and compared with first and second reference voltages. Comparison with the first voltage produces a "signal arrival" pulse, if the received signal exceeds the first reference voltage. Counting of pulses synchronized with the received signal then occurs for a predetermined number of counts.
If the received signal is attenuated by the medium unduly the received signal will not exceed the first reference voltage and the "arrived" signal will be delayed until a succeeding half-cycle of amplitude greater than the first reference voltage occurs. This delay results in measurement errors which are undesirable.
Therefore, it is an object of the present invention to overcome the general disadvantages of the prior art devices.
It is a further object of this invention to provide a transit-time measurement instrument which gives accurate measurements despite random attenuation, in the measurement medium, of the acoustical signal passing therethrough.